Utilization of nutrients is key to many diseases, including obesity, insulin resistance/diabetes mellitus, hyperlipidemia, and others. The capacity to oxidize dietary fat relative to the tendency to store ingested fat, for example, is considered to be a central determinant of susceptibility to dietary fat-induced obesity. Similarly, the capacity to store or oxidize dietary glucose is a key element in insulin resistance and glucose intolerance/diabetes. Tools for assessing the fate of nutrients in the body in living organisms have lagged behind, however. Currently available tools suffer from many limitations.
The oral glucose tolerance test (OGTT) is widely used in medical research and clinical medicine for assessing insulin sensitivity of tissues. The principle of the OGTT is that uptake of glucose from blood by tissues, along with suppression of release of endogenously produced glucose into blood from tissues, is reflected in the clearance rate of an exogenous glucose load from the bloodstream. This approach is crude, however, and no information is generated about the specific metabolic fate or consequences of the glucose administered. As a result, no information is generated about the mechanisms underlying impaired glucose tolerance. Though widely used in clinical practice, the OGTT is of limited utility.
Fat tolerance testing has a similar basis and similar limitations as OGTT. The fat tolerance test measures the uptake of fatty acids from blood by tissues. This approach is also crude, and gives no information about the specific metabolic fate or consequences of the fat administered. As a result, no information is generated about the mechanisms underlying impaired fat tolerance. Fat tolerance testing has mostly been used to assess the clearance of dietary fat from blood in context of evaluating hyperlipidemia. Fat tolerance testing is not helpful for assessing sensitivity to high fat-induced obesity.
Indirect calorimetry (IC), or the measurement of fuel oxidation based on respiration, is useful for whole body studies. IC, however, is expensive and requires complex equipment for small animal studies. Also, IC only reveals the net oxidation of fuels in the whole body, without revealing more details concerning the fate of individual fuels in the tissues.
Insulin/glucose clamps and other intensive approaches are of limited practical utility in clinical practice or broad-based drug screening/discovery, due to their labor-intensive nature. Physiologic relevance is often also uncertain, since the procedures used (e.g. intravenous glucose infusion at high rates) do not mimic normal physiologic intake of these nutrients.
The most direct approach is by use of isotopic techniques. These have been highly problematic, however. The oxidation of 13C- or 14C-labeled glucose or fatty acids to 13CO2 or 14CO2 has been used as a marker of tissue oxidation (1-3). The references cited herein are listed at the end of the specification before the claims. The serious flaws with this approach have been discussed previously (4). In brief, recovery of labeled CO2 is a highly variable and unreliable index of tissue production of CO2, due to re-utilization/exchange pathways of 13CO2 or 14CO2. Yield of labeled CO2 generated oxidatively in tissues can be as low as 20%, or as high as 80% (1-4).
The most common risk factor setting for cardiovascular disease is the so-called syndrome X or multiple risk factor syndrome (15) wherein an individual exhibits the combination of obesity, hypertension, hyperlipidemia, and glucose intolerance or diabetes. This syndrome is now widely believed to be tied together pathogenically by insulin resistance, defined as lower-than-normal sensitivity of tissue to the effects of insulin on glucose metabolism (15).
A primary component of tissue insulin resistance is impairment of the efficiency and rate of skeletal muscle and adipose tissue uptake and metabolism of glucose in response to insulin exposure. One component of tissue glucose metabolism is storage as glycogen; the main alternative pathway for glucose metabolism in a tissue is glycolytic metabolism, leading to oxidation or other fates (FIGS. 2 and 3). Both the storage (non-oxidative) and glycolytic (oxidative) pathways are impaired in insulin resistant tissues, such as skeletal muscle (15).
Because the insulin resistance syndrome is so common—indeed is the most common medical abnormality in contemporary Western populations—a reliable laboratory test for diagnosing and monitoring insulin resistance has long been a very high priority. Various commentators have stated that a clinical marker of insulin resistance would be a “holy grail in the fields of modern diabetes and cardiovascular disease” (C. Kahn, M.D., Director of Scientific Sessions, American Diabetes Association, October 2003). The availability of a clinical test for insulin resistance would affect not only patient care but also would allow drugs to be developed specifically to treat insulin resistance.
Unfortunately, no current laboratory test is a reliable measure of insulin resistance. Serum insulin concentrations are highly variable from assay to assay and are influenced by insulin clearance as well as tissue sensitivity to insulin. Other measures, such as blood triglyceride concentration, fasting glucose concentration, oral glucose tolerance, body mass index, waist-to-hip ratio, etc. correlate poorly with clinical insulin sensitivity (as measured by a labor-intensive research test, such as the insulin-glucose clamp technique; see Ref. 15).
A technique for quantifying glucose metabolism by tissues—in particular, glycolysis and/or glycogen storage of a glucose load—would therefore have enormous impact on medical practice and drug trials.